A variational Monte Carlo approach for core excitations

TL;DR

This paper introduces a systematic variational Monte Carlo method for accurately calculating core excitations in molecules, effectively capturing orbital relaxations and maintaining near-nuclear accuracy with small wave function expansions.

Contribution

It presents a new, straightforward protocol for core excitation calculations that improves upon existing methods by explicitly balancing ground and excited state accuracies.

Findings

Achieves accuracy comparable to top theoretical methods in water, ammonia, and methane.

Effectively captures strong orbital relaxations in core states.

Maintains near-nuclear accuracy during excitations.

Abstract

We present a systematically-improvable approach to core excitations in variational Monte Carlo. Building on recent work in excited-state-specific Monte Carlo, we show how a straightforward protocol, starting from a quantum chemistry guess, is able to capture core state's strong orbital relaxations, maintain accuracy in the near-nuclear region during these relaxations, and explicitly balance accuracy between ground and core excited states. In water, ammonia, and methane, which serve as prototypical representatives for oxygen, nitrogen, and carbon core states, respectively, this approach delivers accuracies on par with the best available theoretical methods even when using relatively small wave function expansions.